The term "multichannel systems" is understood to mean communications systems for common transmission of a plurality of message signals via a common medium, utilizing common devices. As a rule, they are intended for long distances, for which they are especially attractive from both a technical and an economic standpoint. In these cases, depending on the nature of the signals transmitted, a greater or lesser number of intermediate regenerators or intermediate amplifiers (hereinafter referred to by the more general term repeater stations) is needed between the source and the sink of the message signals. Multichannel systems are known from electrical communications technology, in the form of frequency-division multiplexing or carrier frequency systems, and in optical communications they are known in the form of wavelength multiplexing systems.
It is usual in multichannel systems to attach signals of particular frequencies or wavelengths to message blocks (in electrical frequency-division multiplexing technology, these attached signals are often called a pilot tone or pilot frequency; in optical communications they are called a monitoring signal), and these signals take on various auxiliary tasks, such as that of representing the level state of the total band; in this case, the level of these signals is evaluated and used to regulate the level of the total bandwidth, and/or to ascertain the exceeding of certain impermissible level states, such as a drop in the level below a predetermined value. However, they can also be used as a carrier signal, for carrying monitoring information onward or for delivering control instructions to repeater stations. It will readily be appreciated that the two task areas can also be combined; in addition, still other special tasks (such as being utilized as a reference frequency signal) are also common. All of these monitoring or control signals will hereinafter be referred to as secondary signals, in which (temporarily, or on a long-term basis) only one message channel is provided.